Method and apparatus for efficient channel state information dissemination for MU-MIMO transmission schemes based on outdated channel state information

ABSTRACT

A method and apparatus is disclosed herein for channel state information dissemination for multi-user (MU) multiple-input multiple-output (MIMO) (MU-MIMO) transmission schemes based on outdated channel state information. In one embodiment, a method for disseminating channel state information (CSI) coefficients from a first user terminal, where each CSI coefficient corresponds to the CSI previously observed by the first user terminal during transmission resource blocks used by a base station to communicate information to a second set comprising at least one user terminal and not including the first user terminal, comprises: broadcasting by the base station the channel state information (CSI) of each user terminal in the second set of user terminals, on the subset of transmission resources used by the base station to communicate information to each user terminal in the second set; receiving observations of these transmissions by the first user terminal; performing, at first user terminal, eavesdropper channel selection of a subset of CSI coefficients between the base station and the eavesdropping first-user terminal for uplink CSI feedback, the subset chosen out of the set of CSI coefficients between base station and the first user-terminal on the transmission resources used by the base station to communicate information to each user terminal in the second set; scheduling the subset of the selected CSI coefficients for uplink transmission; and transmitting the subset of the selected CSI coefficients via the uplink transmission.

PRIORITY

The present patent application is a divisional of U.S. patentapplication Ser. No. 14/425,302, having a 371(c) date of Mar. 14, 2013,which is a U.S. National Phase Application under 35 U.S.C. §371 ofInternational Application No. PCT/US2013/031719, filed Mar. 14, 2013,entitled METHOD AND APPARATUS FOR EFFICIENT CHANNEL STATE INFORMATIONDISSEMINATION FOR MU-MIMO TRANSMISSION SCHEMES BASED ON OUTDATED CHANNELSTATE INFORMATION, which claims priority to and incorporates byreference the corresponding provisional patent application Ser. No.61/699,729, titled, “Method and Apparatus for Efficient Channel StateInformation Dissemination for MU-MIMO Transmission Schemes based onOutdated Channel State Information,” filed on Sep. 11, 2012.

FIELD OF THE INVENTION

Embodiments of the present invention relate to the field of multiuserMultiple Input Multiple Output (MIMO) wireless transmission systems;more particularly, embodiments of the present invention relate to a newclass of Multiuser-MIMO (MU-MIMO) techniques that exploit completelyoutdated channel state information at the transmitter (CSIT).

BACKGROUND

Many recent advances in wireless transmission have rested on the use ofmultiple antennas for transmission and reception. Multiple antennas,fundamentally, can provide an increase in the numbers of Degrees ofFreedom (DoFs) that can be exploited by a wireless system fortransmission, i.e., the number of scalar data streams that can besimultaneously transmitted to the receiving parties in the system. Here,DoFs can be used to provide increased spectral efficiency (throughput)and/or added diversity (robustness). Indeed, a Single User MIMO(SU-MIMO) system with N_(t) transmission antennas serving a single userterminal (UT) with N_(r) receive antennas may be able to exploit up tomin(N_(t), N_(r)) DoFs for downlink transmission. These DoFs, can undercertain conditions be used to improve throughput by a factor that growslinearly with min(N_(t), N_(r)). Such benefits of MIMO, and increasedDoFs, are behind much of the interest in using MIMO in new and futuresystems.

Exploiting such DoFs often requires some amount of cost to the system.One such cost is knowledge of the channel state between transmitting andreceiving antennas. Such Channel State Information (CSI) often has to beavailable to either the transmitter (such CSI is termed CSIT) and/or tothe receiver (such CSI is termed CSIR). The DoFs available also dependon having sufficient “richness” in the channels between transmitting andreceiving antennas. For example, SU-MIMO CSIR-based systems such as BitInterleaved Coded Modulation (BICM) and D-BLAST can achieve the maximumpossible DoFs of min(N_(r), N_(r)) under suitable channel conditions.Under such conditions, they can be used to provide corresponding linearincreases in spectral efficiency. Such designs are well understood bythose familiar with the state of the art.

Similarly, a Multi-User MIMO (MU-MIMO) system with N_(t) transmissionantennas at the base station (BS) and K single-antenna users (N_(r)=1)can provide up to min(N_(r), K) DoFs. As in the case of SU-MIMO, MU-MIMOcan be used to improve throughput linearly with min(N_(r), K).

However, unlike SU-MIMO, many MU-MIMO techniques (in fact most if notall of the prevailing MU-MIMO techniques used and studied for standards)require knowledge of CSIT. MU-MIMO based on CSIT, unlike SU-MIMO basedon CSIR, requires additional overheads to estimate CSI and feedback CSIto transmitters before the transmission can even take place (see Caireet al., “Multiuser MIMO achievable rates with downlink training andchannel state,” in IEEE Transactions on Information Theory, June 2010,pp. 2845-2866). Despite such overheads, MU-MIMO is of practical interestsince it has the benefit over SU-MIMO of being able to grow the DoFswithout having to add many receive antennas, radio frequency (RF)chains, or increase processing (e.g., decoding) complexity to portableor mobile devices.

The issue of CSI overhead is a fundamental issue that should not beoverlooked in assessing such conventional MIMO. In fact, suchCSI-related overhead can represent a fundamental “dimensionalitybottleneck” that can limit the net spectral efficiency increase that canbe obtained with conventional CSI-dependent MIMO. In particular, if onewants to continue to exploit the growth in DoFs (e.g., linear growth) byincreasing N_(t) (or N_(r) or K), one also has to consider how tosupport increased system overhead in obtaining the CSI required toformulate transmissions and decode at the receivers. Such overhead caninclude increased use of the wireless medium for pilots supporting CSIestimation and increased feedback between receiving and transmittingentities on such CSI estimates. As an example, assume that for eachcomplex scalar value that defines the CSI between a single TX antennaand a single RX antenna (this type of CSI is often termed direct CSI bysome in the Standards community) a fixed percentage F_(csi) ofwireless-channel resources is dedicated to pilots and/or feedback. Onecan easily see that as the dimension of the CSI required scales withquantities like N_(t), N_(r) and/or K, the total CSI system-relatedoverhead grows (e.g., by N_(t)×F_(csi)). For example, for K singleantenna users, each with N_(t) CSI scalar terms with respect to thetransmitting antenna, there are KNT such scalars. Supporting an increasein the dimension of the CSI can take more wireless-channel resources,and reduces the amount of resources left for data transmission. Thisoverhead increase can limit continued growth in throughput if spectralefficiency improvements do not offset increased CSI overheads. Thus,although symbols representing coded data information are used moreefficiently, with increased robustness and/or spectral efficiency due tothe increased DoFs by MIMO, the net spectral efficiency increases haveto account for the CSI overhead. Thus, the net spectral efficiencygrowth is in fact less than that of individual data symbols as only afraction of no more than (1−N_(t)×F_(csi)) of symbols can be used fordata.

Recently, a new class of MU-MIMO techniques has emerged, which takeadvantage of outdated CSIT to enable increases in DoFs via “InterferenceAlignment” (IA) at each of the receivers. These are referred as “MAT”schemes, named after the initials of the inventors of this family ofschemes. These schemes exploit multi-round transmission sessions,referred to as “MAT” sessions. Each round in a MAT session comprises oneor more “slots” or resource blocks, over which each user equipment (UE)possibly experiences distinct channels.

What is attractive about these schemes is that the required CSIT isallowed to be fully outdated. In particular, these schemes enable DoFgains by only exploiting knowledge of past channels and rely on noknowledge of the current channel state at the transmitter (i.e., noknowledge at the transmitter is required about the user channels overwhich transmission is about to take place). This is in sharp contrast toconventional MU-MIMO systems, whose efficacy intimately depends on theaccuracy of the CSIT at the time of the data transmission. That is, theefficacy of conventional MU-MIMO intimately depends on how accuratelythe transmitter knows a priori the channels over which data transmissionin MU-MIMO is to take place. It is possible for a Multi-User MIMO(MU-MIMO) system with N_(t) transmission antennas at the BS and Lsingle-antenna users to achieve K/(1+1/2+1/3+ . . . +1/K) DoFs withoutdated CSIT, where K=min(N_(t), L). As K grows, the system DoFs growas K/(γ+log(K)), where γ is the Euler-Mascheroni constant and is numberbetween 0.57 and 0.58.

MU-MIMO schemes based on outdated CSI at the transmitter have someinherent challenges and limitations in the scenarios in which they areused. The first inherent issue is that they often require high Signal toNoise Ratios (SNR) to operate effectively. For example, the originalinterference alignment (IA) scheme may require up to 20 dB of SNR. Thisis due to a property of the interference alignment process, whichresults in noise being amplified in the resulting interference-alignedstreams. As a consequence of this, the original IA technique has limitedapplication to many users in a cellular environment. For example,cell-edge users in conventional cellular often experienceSignal-to-Interference-plus-Noise-Ratios (SINR) on the order of 0 dB orless, due to the interference coming from interfering cells not servingthe K users. Many users, not just cell-edge users, do not have SINRs onthe order of 20 dB or more. Because these schemes however can rely oncompletely outdated CSI, these schemes have less stringent requirementsfor user scheduling based on the collected CSIT.

Other techniques have been proposed for scheduling and MU-MIMOtransmission based on outdated channel state information. One techniqueenables scheduling MAT sessions with improved rate-performance benefits.Other techniques, referred to as packet-centric eavesdropper-schedulingembodiments, enable even higher scheduling benefits, by scheduling ofmulti-round multiuser transmissions, which conform to the MAT sessionprinciples, without comprising MAT sessions.

Much like their conventional MU-MIMO counterpart, MU-MIMO schemes basedon outdated CSI have CSI dissemination requirements. First, theseschemes also require CSIT, that is, the CSI of eavesdroppinguser-terminals, i.e., terminals eavesdropping on the transmission ofinformation to one or more other terminals, is needed at the basestation, to enable MU-MIMO transmission in later-round transmissions.Their raw CSIT overheads also increase in the presence of scheduling, aseavesdropper CSI is needed from more user terminals during each slot toallow choosing a “good” eavesdropper and enabling scheduling benefits.

Unlike their conventional MU-MIMO counterparts, MU-MIMO schemes withoutdated CSI (with and without scheduling) also require CSIRdissemination, that is, CSI of eavesdropping user-terminals needs to bemade available to the user terminals for which the transmissions wereintended, so as to allow these terminals to perform coherent detectionand decoding.

SUMMARY OF THE INVENTION

A method and apparatus is disclosed herein for channel state informationdissemination for multi-user (MU) multiple-input multiple-output (MIMO)(MU-MIMO) transmission schemes based on outdated channel stateinformation. In one embodiment, a method for disseminating channel stateinformation (CSI) coefficients from a first user terminal, where eachCSI coefficient corresponds to the CSI previously observed by the firstuser terminal during transmission resource blocks used by a base stationto communicate information to a second set comprising at least one userterminal and not including the first user terminal, comprises:broadcasting by the base station the channel state information (CSI) ofeach user terminal in the second set of user terminals, on the subset oftransmission resources used by the base station to communicateinformation to each user terminal in the second set; receivingobservations of these transmissions by the first user terminal;performing, at first user terminal, eavesdropper channel selection of asubset of CSI coefficients between the base station and theeavesdropping first-user terminal for uplink CSI feedback, the subsetchosen out of the set of CSI coefficients between base station and thefirst user-terminal on the transmission resources used by the basestation to communicate information to each user terminal in the secondset; scheduling the subset of the selected CSI coefficients for uplinktransmission; and transmitting the subset of the selected CSIcoefficients via the uplink transmission.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood more fully from the detaileddescription given below and from the accompanying drawings of variousembodiments of the invention, which, however, should not be taken tolimit the invention to the specific embodiments, but are for explanationand understanding only.

FIG. 1 is a block diagram of one embodiment of a base station.

FIG. 2 is a block diagram of one embodiment of user equipment (UE).

FIG. 3 is a timing diagram of a two-UE MAT session.

FIG. 4 is a timing diagram illustrating CSIT/CSIR dissemination by useof orthogonal feedback in the uplink (UL) and downlink (DL)respectively.

FIG. 5 is a timing diagram illustrating three UEs involved in two-userMAT sessions.

FIG. 6 is a timing diagram illustrating CSIT dissemination for a commonMAT session by MU-MIMO data transmission in the UL.

FIG. 7 is a timing diagram illustrating CSIT dissemination of CSIpertaining to a common intended receiver by MU-MIMO data transmission inthe UL.

FIG. 8 is a timing diagram illustrating CSIT UL feedback with UE to UEfeedback (FB).

FIG. 9 is a timing diagram illustrating CSIT FB via UL MU-MIMO servingmore users in the UL MU-MIMO than the corresponding MAT session.

FIG. 10 is a timing diagram illustrating a way to disseminate CSIR inthe DL.

FIG. 11 is a timing diagram illustrating CSIR dissemination of round-2eavesdropper CSI based on scalar DL transmissions.

FIG. 12 is a timing diagram illustrating round-1 eavesdropper CSIR viaDL MU-MIMO based on outdated CSI.

FIG. 13 is a timing diagram illustrating UE-based selective eavesdropperCSIT feedback between a UE and a base station (BS).

FIG. 14 is a flow diagram of one embodiment of a process for sendingUE-based selective eavesdropper CSIT feedback at a UE.

FIG. 15 is a flow diagram of one embodiment of a process for sendinguser-terminal based selective eavesdropper CSIT feedback at a UE.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

Embodiments of the invention include methods and apparatuses forefficient delivery of CSIT and CSIR, so as to enable MU-MIMO based onoutdated CSI. These techniques can be also readily used for enablingMU-MIMO based on partially outdated CSI. MU-MIMO is a future radioaccess technology that enables multiplexing gains in environments thatinvolve serving mobile users with delay tolerant traffic.

Although existing control information dissemination methods can be usedto disseminate channel state information needed for enabling MU-MIMObased on outdated CSI, as explained herein, these fail to exploit theside information available at each user terminal, and the relaxed timingconstraints associated with these inventions. Furthermore, the proposedtechniques can offer enormous reductions in the required CSI overheads.As described herein, embodiments of the invention can providesubstantial efficiency benefits with respect to existing controlinformation dissemination methods.

Also disclosed are efficient CSI dissemination techniques for enablingscheduling benefits in MU-MIMO based on outdated CSI. Embodiments of thepresent invention also provide efficient methods for terminal-drivenselective eavesdropper-CSIT transmission for MU-MIMO embodiments withscheduling. This reduces the CSIT overhead, as part of the user-channelselection for scheduling is performed at the mobiles prior to CSITtransmission.

In the following description, numerous details are set forth to providea more thorough explanation of the present invention. It will beapparent, however, to one skilled in the art, that the present inventionmay be practiced without these specific details. In other instances,well-known structures and devices are shown in block diagram form,rather than in detail, in order to avoid obscuring the presentinvention.

Some portions of the detailed descriptions that follow are presented interms of algorithms and symbolic representations of operations on databits within a computer memory. These algorithmic descriptions andrepresentations are the means used by those skilled in the dataprocessing arts to most effectively convey the substance of their workto others skilled in the art. An algorithm is here, and generally,conceived to be a self-consistent sequence of steps leading to a desiredresult. The steps are those requiring physical manipulations of physicalquantities. Usually, though not necessarily, these quantities take theform of electrical or magnetic signals capable of being stored,transferred, combined, compared, and otherwise manipulated. It hasproven convenient at times, principally for reasons of common usage, torefer to these signals as bits, values, elements, symbols, characters,terms, numbers, or the like.

It should be borne in mind, however, that all of these and similar termsare to be associated with the appropriate physical quantities and aremerely convenient labels applied to these quantities. Unlessspecifically stated otherwise as apparent from the following discussion,it is appreciated that throughout the description, discussions utilizingterms such as “processing” or “computing” or “calculating” or“determining” or “displaying” or the like, refer to the action andprocesses of a computer system, or similar electronic computing device,that manipulates and transforms data represented as physical(electronic) quantities within the computer system's registers andmemories into other data similarly represented as physical quantitieswithin the computer system memories or registers or other suchinformation storage, transmission or display devices.

The present invention also relates to apparatus for performing theoperations herein. This apparatus may be specially constructed for therequired purposes, or it may comprise a general purpose computerselectively activated or reconfigured by a computer program stored inthe computer. Such a computer program may be stored in a computerreadable storage medium, such as, but is not limited to, any type ofdisk including floppy disks, optical disks, CD-ROMs, andmagnetic-optical disks, read-only memories (ROMs), random accessmemories (RAMs), EPROMs, EEPROMs, magnetic or optical cards, or any typeof media suitable for storing electronic instructions, and each coupledto a computer system bus.

The algorithms and displays presented herein are not inherently relatedto any particular computer or other apparatus. Various general purposesystems may be used with programs in accordance with the teachingsherein, or it may prove convenient to construct more specializedapparatus to perform the required method steps. The required structurefor a variety of these systems will appear from the description below.In addition, the present invention is not described with reference toany particular programming language. It will be appreciated that avariety of programming languages may be used to implement the teachingsof the invention as described herein.

A machine-readable medium includes any mechanism for storing ortransmitting information in a form readable by a machine (e.g., acomputer). For example, a machine-readable medium includes read onlymemory (“ROM”); random access memory (“RAM”); magnetic disk storagemedia; optical storage media; flash memory devices; etc.

Overview

Embodiments of the invention include methods and apparatuses forefficient channel state information dissemination in order to enableMU-MIMO transmission based on outdated CSI. In one embodiment, thetechniques set forward in embodiments of the invention are used toenable the original MAT scheme discussed above, scheduling-based MU-MIMOwith outdated CSI, as well as the more recent methods for MU-MIMO basedon partially outdated CSI.

Similar to conventional MU-MIMO, schemes relying on outdated CSI alsorequire channel state-information (CSI) at the transmitter (CSIT). Inconventional MU-MIMO, CSIT is required to design the precoder to be usedfor MU-MIMO transmission. The efficacy of the MU-MIMO transmission isdictated by how accurately the CSIT, collected during training (e.g.,during the transmission of reference signaling), describes the channelduring the time of MU-MIMO transmission. As a result, in conventionalMU-MIMO, the CSIT collected during reference signaling at each UE(considered for MU-MIMO transmission), must be fed back to the basestation in a very timely fashion, i.e., within the coherence time of thechannel, so that it accurately describes the user channels experiencedduring the MU-MIMO data transmission phase. MU-MIMO methods also requireaccurate CSIT. However, the accuracy of CSIT is measured with respect tothe channels where CSI was collected, and not with respect to the futureinstances where this CSIT will be used for transmission at the basestation. As a result, the strict timing constraints of conventionalMU-MIMO in delivering CSIT are not present with outdated CSI. Thisflexibility allows for more options in delivering CSIT for outdated CSIschemes.

In MU-MIMO methods based on outdated CSI, the CSIT, and in particular, achannel coefficient made available to the base station from any givenUE, may also need to be made available to one or more of the other UEs.This form of CSI at the receiver (CSIR) is needed to enable such UEs toperform coherent decoding of the messages intended for them. Similarlyto CSIT, CSIR dissemination has no timing constraints, other than thetiming constraints of the transmitted message sources. In the presenceof delay-tolerant traffic (where outdated CSI techniques would beattractive), there is significant flexibility in delivering CSIR.Furthermore, the fact that each user has a piece of CSIR needed by otherusers can be exploited for further efficacy improvements in the CSIRdissemination.

Note that for purposes herein, the terms user equipment (UE) and userterminal (UT) are used interchangeably.

FIG. 1 and FIG. 2 are block diagrams of one embodiment of a basestation/eNB 100 and UE 200, respectively.

Referring to FIG. 1, in one embodiment, base station 100 serves one ormore cells and is equipped with N_(t) antennas, 135 a through 135 t. InFIG. 2, in one embodiment UE 200 is equipped with N_(R) antennas, 235 athrough 235 r.

Referring back to FIG. 1, base station 100 includes a transmit processor115 that receives data for one or more UE from a data source 110,processes the data for each UE, and transmits data for each UE. In oneembodiment, processor 115 also receives and processes information from acontroller processor 170 and provides control symbols. In oneembodiment, processor 115 also generates reference symbols for one ormore reference signals. A transmit (TX) MIMO processor 120 performsprecoding on the data symbols, the control symbols, and/or the referencesymbols for each UE based on one or more precoding vectors determinedfor that UE. In one embodiment, processor 120 provides (up to) N_(t)output streams, one to each of the modulators (MODs) in modules 130 athrough 130 t. Each modulator 130 processes its respective stream (e.g.,for OFDM, etc) to obtain an output sample stream. Each modulator 130further processes (e.g., convert to analog, amplify, filter, upconvert,etc.) the output sample stream to obtain a downlink signal. Up to N_(t)output streams from modulators 130 a through 130 t are transmitted viaN_(t) antennas 135 a through 135 t, respectively.

Referring to FIG. 2, in one embodiment, at UE 200, antennas 235 athrough 235 r receive downlink signals from base station 100, other basestations, as well as uplink transmission signals from other UEs.Antennas 235 a through 235 r provide received signals to demodulators(DEMODs) 230 a through 230 r, respectively. In one embodiment, eachdemodulator 230 conditions (e.g., amplify, downconvert, digitize) arespective received signal to obtain input samples. In one embodiment,each demodulator 230 further processes the input samples (e.g., forOFDM, etc) to obtain received samples. A receive processor 245 processes(e.g., demodulate and decode) the detected symbols, provides decodeddata for UE 200 to a data sink 250, and provides decoded controlinformation to a control processor 270. In accordance with oneembodiment of the invention, receive processor 245 also processes (e.g.,demodulate) samples of transmissions not intended for UE 200 andprovides the samples to a control processor 270. Control processor 270stores the samples in a memory module 260.

A channel processor 280 at UE 200 estimates the channel response foreach cell of interest and provides a channel matrix for the cell.Processor 270 and/or 280 determines channel information based on channelmatrix for each cell of interest. In accordance with one embodiment ofthe invention, this operation also occurs on transmission resources notallocated to transmission of data for UE 200. In that case, processor280 stores determined channel matrix in memory module 260, for lateruse.

On the uplink, at UE 200, a transmit processor 215 receives data from adata source 210 and control information from controller/processor 270.Processor 215 processes (e.g., encode and modulate) the data and controlinformation to obtain data symbols and control information respectively.In one embodiment, processor 215 also generates reference symbols forone or more reference symbols. In one embodiment, the symbols fromtransmit processor 215 are precoded by a TX MIMO processor 220 ifapplicable, further processed by modulators 230 a through 230 r, andtransmitted to base station 100, and possibly other base stations, andpossibly other UEs receiving uplink transmissions.

Referring back to FIG. 1, at base station 100, the uplink signals fromUE 200 and other UEs are received by antennas 135, processed bydemodulators 130, detected by a MIMO detector 140 and further processedby a receive processor 145 to obtain decoded data and controlinformation sent by UE 200 and other UEs. Processor 145 provides thedecoded data to a data sink 150 and the decoded control information tocontroller/processor 170. In one embodiment, processor 145 also provides“analog-valued” (digitized samples with e.g., up to floating pointprecision) control information transmitted by one or more UEs,indicative of channel state information between one or more antennaelements at the UEs and one or more antennas at the base station.

A channel processor 180 at base station 100 estimates the channelresponse from UE 200 and other UEs of interest and provides a channelmatrix for each UE. In one embodiment, processor 170 and/or 180determines channel information based on channel matrix for each UE ofinterest. In accordance with one embodiment of the invention, processor180 stores determined channel matrix in memory module 160, for lateruse.

In one embodiment, scheduler 190 schedules UEs for data transmission onthe downlink and/or uplink. Scheduler 190 and/or other processors andmodules at base station 100 may perform processes for the techniquesdescribed herein. These include, scheduling transmission of controlinformation in the uplink by UE 200 and other UEs, in a manner thatenables the efficient control information dissemination required toenable MU-MIMO transmission based on outdated CSI with or withoutscheduling benefits. Some embodiments of this invention can be enabledby a scheduler/processor module 290 at UE 200. This module may performprocesses for the techniques described herein, including embodiments ofthe inventions that are associated with terminal-driven selectivefeedback of control information pertaining to channel matrices at UE 200during transmission resources dedicated to transmission to UEs otherthan UE 200.

Controllers/processors 170 and 270 direct the operation at base station100 and UE 200, respectively. Memories 160 and 260 may store data andprogram codes for base station 100 and UE 200, respectively.

FIG. 3 is a timing diagram associated with the signaling required toenable a MAT session between two users, based on outdated CSI. AssumingN_(r)≧(2N_(t)). This scheme can enable sending 2N_(t) symbols to each UEover 3 time-frequency data-transmission time-frequency slots, for atotal multiplexing gain of 4N_(t)/3 per slot. In particular, the schemecommunicates to UE k a vector (sequence) x_(k) of dimensions 2N_(t)×1over three slots (or resource blocks). Without loss of generality weconsider the case N_(t)=1, in which case N_(r)≧2, and x_(k) is2-dimensional. As shown in FIG. 3, reference signals (e.g., DL pilots)are transmitted along with the desired transmission signal x_(k) duringtime-frequency resource block (t_(k), f_(k)), for each k=1, 2. Notethat, for each k=1, 2, the reference signals transmitted duringtime-frequency resource block (t_(k), f_(k)) allow both the intendedreceiver (UE k) and the eavesdropper (UE j=3−k) to estimate theirchannels. As shown in FIG. 3, the eavesdropper channel in this cycle isrepresented via g_(j→k) signifying that this is the channel of(eavesdropper) j during the transmission of a message for user k.

To enable this 2-user MAT scheme, the user terminal channel needs to bemade available to the base station (CSIT) so as to enable thesecond-round transmission, which occurs during time-frequency resourceblock (t₁₂, f₁₂). This is accomplished via the uplink (UL) feedback (FB)cycle, which takes place during (t^(UL), f^(UL)) time-frequency resourceblocks. Each of the eavesdropper channels, h_(j→k), also needs to bemade available to UE receiver k (CSIR), in order to enable coherentdecoding UE k. This is accomplished by use of a downlink (DL) FB cycle,which takes place during (t^(DL), f^(DL)) time-frequency resourceblocks. The protocols for the UL and DL transmissions within the(t^(UL), f^(UL)) and the (t^(DL), f^(DL)) time-frequency resource blocksare subjects of embodiments of inventions described herein.

FIG. 4 a timing diagram illustrating CSIT and CSIR dissemination byorthogonal scalar signaling in the UL and DL, respectively. This servesas a reference design for CSI dissemination. As shown in FIG. 4, in thereference design, distinct UL time-frequency resources are allocated toeach UE for training (e.g., for sending UL pilots from each UE in orderto allow the base station to estimate the channel from each UE to thebase station) and for transmission of their eavesdropper channelcoefficients to the base station. Similarly, the base station also usesdistinct time-frequency resources for delivering the eavesdropper CSI toeach intended receiver. In the example shown in FIG. 4, the base stationtransmits DL pilots during time-frequency block (t^(DL), f^(DL)) from asingle antenna. This allows each UE to estimate the channel between thepilot-transmitting antenna and the UE antenna. The base station alsobroadcasts (from the pilot-transmitting antenna) scalar channelcoefficients intended for UE1 and UE2 respectively on non-overlappingsubsets of resource elements (within the same time-frequency block).Embodiments of the invention include techniques that have higher CSIdissemination efficiency (in terms of the UL/DL resources utilized) thanthe reference design shown in FIG. 4

First, note that, although FIG. 3 shows a two-user MAT session betweentwo UEs, i.e., UE 1 and UE 2, in general each UE may be engaged in MATsessions with several users. This for instance would be the caseinvolving scheduling. One such example is shown in FIG. 5. The basestation employs a MAT session between UE 1 and UE 2 and involves sendingx₁(k) to UE1 and x₂(k) to UE2. As shown in FIG. 5, reference signals (DLpilots) are transmitted along with the desired transmission signalx_(j)(k) during time-frequency resource block (t_(j)(k), f_(j)(k)), foreach j=1, 2. Note that, for each j=1, 2, the reference signalstransmitted during time-frequency resource block (t_(j)(k), f_(j)(k))allow both the intended receiver (UE j) and the eavesdropper (UE j′=3−j)to estimate their channels.

Also as shown in FIG. 5, another MAT session is employed by the basestation in between UE 1 and UE 3 and involves sending y₁(k) to UE 1 andy₃(k) to UE3. Reference signals (DL pilots) are transmitted along withthe desired transmission signal y_(j)(k) for j=1 and j=3, duringtime-frequency resource block (t₃(k), f₃(k)), and (t₄(k), f₄(k)),respectively. Note that, for each j=3, 4, the reference signalstransmitted during time-frequency resource block (t_(j)(k), f_(j)(k))allow both UE 1 and UE 3 estimate their channels.

Furthermore, as shown in FIG. 5, there is also a third two-user MATsession employed by the base station and involves sending z₂(k) to UE 2and z₃(k) to UE 3. Reference signals (DL pilots) are transmitted alongwith the desired transmission signal z_(j)(k) for j=2 and j=3, duringtime-frequency resource block (t₅(k), f₅(k)), and (t₆(k), f₆(k)),respectively. Note that, for each j=5, 6, the reference signalstransmitted during time-frequency resource block (t_(j)(k), f_(j)(k))allow both the UE 2 and UE 3 to estimate their channels.

Also shown in FIG. 5 (in the right column) is the eavesdropper channelin each time-frequency block (t_(j)(k), f_(j)(k)), whereby h_(m→n)signifies that this is the channel of (eavesdropper) m during thetransmission of a message for user n. As FIG. 5 illustrates, resourcescan be aggregated in both the UL and the DL for efficient CSIT (UL) andCSIR (DL) dissemination. Specifically, FIG. 5 shows an UL FB cycle,which takes place during (t^(UL), f^(UL)) time-frequency resourceblocks. Each of the eavesdropper channels, h_(j→k), also needs to bemade available to UE receiver k (CSIR) in order to enable coherentdecoding UE k. This is accomplished by use of a DL FB cycle, which takesplace during (t^(DL), f^(DL)) time-frequency resource blocks.

CSIT Dissemination via UL Transmissions

In one embodiment, CSIT dissemination occurs via uplink transmissions.Such information is used for transmitting information in a multi-user(MU)-MIMO wireless communication system having a multi-antennatransmitter and a plurality of user terminals. In one embodiment, amethod for disseminating CSIT via uplink transmissions includes sendinga pilot to a base station from each of at least two user terminals aspart of round-1 MAT transmissions to train antennas at the base station,eavesdropping, by those user terminals, the round-1 transmissions, andthen simultaneously transmitting to the base station, in a sametime-frequency block, from the those user terminals channel stateinformation corresponding to eavesdropped round-1 transmissions thateach of the at least two user terminals has eavesdropped to enable a MATsession between the at least two user terminals. By doing so, the CSITinformation is disseminated so that round-2 transmission of the MATsession can be performed.

FIG. 6 is a timing diagram illustrating a method for efficientdissemination of the CSIT needed at the base station in order to enablethe (round-2 transmissions of the) MAT sessions in FIG. 3 (and FIG. 5).In one embodiment, within a single time-frequency block, UL pilots aresent orthogonally by two UEs (either on different resources, or by useof an orthogonal code across two or more resources) to train the N_(R)≧2antennas at the base station. In the same time-frequency block, the twoUEs transmit simultaneously (over each data-transmission time-frequencyelement) channel coefficients on round-1 messages that each UE haseavesdropped, and which are required at the base station to enableround-2 transmission. Thus, in one embodiment shown in FIG. 6, users 1and 2 transmit simultaneously channels that are needed to enable a MATsession between the two users.

In one embodiment, at least one of the coefficients transmitted involvesenabling a MAT session between the transmitting UE and a different UE.One such example is shown in FIG. 7 and involves UE 1 and UE 2simultaneously transmitting eavesdropped channels h_(1→3) and h_(2→3),respectively, both pertaining to a common intended receiver, in thiscase, UE 3. This embodiment is advantageous in the case where UE 3 canalso eavesdrop in the UL, as it would also obtain a single linearcombination of two coefficients that it would also need to acquire(CSIR) for coherent decoding. Hence only one of these two coefficientswould need to be disseminated in the DL to UE 3, because the remainingone would be decipherable from the UE 3's UL observation of thecorresponding UL session shown in FIG. 7.

CSIR Dissemination Via UL and DL Transmissions

In one embodiment, CSIR dissemination occurs via uplink and downlinktransmissions. Such information is used for transmitting information ina multi-user (MU)-MIMO wireless communication system having amulti-antenna transmitter and a plurality of user terminals. In oneembodiment, a method for disseminating the CSIR information via uplinkand downlink transmissions includes obtaining, by a first user terminalobserving the uplink channel, a linear combination of eavesdroppedmessages transmitted by multiple user terminals to the base station,where the user terminals do not include the first user terminal; thefirst user terminal estimating the channel between those user terminalsand itself using the linear combination; and generating, by the firstuser terminal, channel coefficients for use in communicating with thebase station.

FIG. 8 is a timing diagram illustrating a method, associated with thetiming diagram of FIG. 5 that relies on UL feedback dedicated to asingle UE as in the embodiment in FIG. 7 in accordance with oneembodiment. In this case, FIG. 8 explicitly shows the intended UE ineach case “listening in” on the uplink channel and obtaining a linearcombination of the eavesdropped messages transmitted by the other twoUEs to the base station.

Referring to FIG. 8, during time-frequency block (t₁ ^(UL), f₁ ^(UL)),UE 2 and UE 3 transmit orthogonal UL pilots. These UL pilots allow thebase station to estimate the channel between the pilot-transmitting UEsand the base station. These UL pilots also allow UE 1 to estimate thechannel between the pilot-transmitting UEs and UE 1. UE j for j=2, 3also transmit, during time-frequency block (t₁ ^(UL), f₁ ^(UL)), channelcoefficient pertaining to channels UE j has experienced during thetransmission of messages intended for UE 1. In the embodimentillustrated in FIG. 8, UE2 and UE3 transmit coefficients simultaneously.Note that, since the base station has at least 2 antennas, on eachresource element, the base station has two observations (noisy linearcombinations) of the two coefficients that were transmitted by UE 2 andUE 3 on that element. As a result, the base station can recover (i.e.,estimate) the two coefficients. On the other hand, UE 3 may only haveone antenna. In that case, UE 3 obtains a single (noisy) linearcombination of the two transmitted coefficients. Although this singlecombination does not suffice in decoding the two coefficients, in oneembodiment, the coefficients of the linear combination and the value ofthe linear combination (on each resource element) are all stored by UE 3for later use. Indeed a single extra equation is only needed by UE 3 toresolve both of these coefficients. For instance, making available onlyone of these two transmitted coefficients by use of other transmissionoptions to UE 3, also makes available the other (missing) coefficient.This can easily be accomplished at UE 3 by exploiting the stored linearcombination and the known coefficient. As FIG. 8 illustrates, theprocess is repeated with UE j for =2, 3 listening in on channelcoefficients sent by the other two UEs during time-frequency block(t_(j) ^(UL), f_(j) ^(UL)). These correspond to coefficients on channelsexperienced by the other two UEs during the transmission of messagesintended for user j.

FIG. 9 is a timing diagram illustrating a method in which two-user MATsessions are enabled to single-antenna users by a base station withN_(t)>2 in accordance with one embodiment. Referring to FIG. 9, the ULfeedback channel can be used to provide CSIT simultaneously from 3 ormore (up to N_(t)) UEs at the same time. As illustrated, duringtime-frequency block (t^(UL), f^(UL)), UE 1, UE 2, and UE 3 transmitorthogonal UL pilots. These UL pilots enable the base station toestimate the channel between each UE and the N_(t) base stationantennas. The three UEs also provide CSIT feedback simultaneously to thebase station, by simultaneously transmitting channel coefficients on asubset of resource elements of time-frequency block (t^(UL), f^(UL)),where each channel coefficient transmitted by UE j pertains to thecoefficient of a channel experienced by UE j during the transmission ofmessage intended for some other user (or users). In one embodiment, theintended receiver is from the same set, i.e., a =2 or 3, b=1 or 3, andc=1 or 2. In one embodiment, a=b=c and these are different from 1, 2,and 3.

CSIR Dissemination via DL Transmissions

In one embodiment, CSIR dissemination occurs via downlink transmissions.Such information is used for transmitting information in a multi-user(MU)-MIMO wireless communication system having a multi-antennatransmitter and a plurality of user terminals. In one embodiment, amethod for disseminating the CSIR information via downlink transmissionsincludes receiving, by each of at least two user terminals, observationsof a pilot transmitted by the base station to antennas of the two userterminals; estimating, by each of the those terminals, a channel betweena pilot-transmitting antenna of a base station and their antenna basedon the observations of the pilot transmitted by the pilot-transmittingantenna; observing a broadcast of a first linear combination of at leasttwo channel coefficients made by the base station based on observationsmade by the base station, where the first linear combination includesestimates of CSI channel coefficients; and at least one of those userterminals obtaining an estimate of channel coefficients based on theobserved first linear combination and based on a combination of itsestimation of the channel between the pilot-transmitting antenna and itsantenna based on the observation the at least one user terminalreceived.

FIG. 10 is a timing diagram illustrating a method in which CSIR isdisseminated in the DL in accordance with one embodiment. This is moreefficient than the embodiment presented in FIG. 4. The embodiment inFIG. 10 transmits scalar coefficients in the DL in a similar manner tothe manner in which data is transmitted in the second round of thereference data-transmission scheme shown in FIG. 3.

Referring to FIG. 10, the base station transmits DL pilots duringtime-frequency block (t^(DL), f^(DL)) from a single antenna. This allowseach of the two UEs to estimate their channel between thepilot-transmitting antenna and their UE antenna within time-frequencyblock (t^(DL), f^(DL)) On (at least one) resource-element not allocatedfor DL pilot transmission within time-frequency block (t^(DL), f^(DL)),the base station broadcasts (from the same antenna from which the pilotwas transmitted within the time-frequency block) the sum of two scalarchannel coefficients, h_(1→2) and h_(2→1). As shown in FIG. 10, UE 1uses its estimate of the channel within time-frequency block (t^(DL),f^(DL)), obtained based on the single-pilot transmission, together withthe single observation of the transmission of the sum of the two channelcoefficients, h_(1→2) and h_(2→1), to recover (estimate) the sum ofh_(1→2) and h_(2→1). Then, UE1 exploits its knowledge of h_(1→2) toobtain (estimate) h_(2→1). In one embodiment, UE 1 estimates h_(2→1) bysubtracting from the sum estimate the value of h_(1→2). Similarly, UE 2recovers (estimates) from the single observation received on itsantenna-element the sum of the two channel coefficients. Then, UE 2exploits its knowledge of h_(2→1) to obtain (estimate) h_(1→2). In oneembodiment, UE 2 estimates h_(1→2) by subtracting from the sum estimatethe value of h_(2→1). Hence, the two user terminals can obtainsimultaneously one CSI coefficient they need based on the CSIcoefficient they have and the received linear combination, yielding anefficiency improvement of a factor of 2 with respect to the originalCSIR dissemination scheme in FIG. 4. The simultaneous DL disseminationof CSIR shown in FIG. 10 is exactly the same operation used in the last(second) data-transmission round of the two-user MAT scheme (see FIG. 3and FIG. 5).

The scalar DL dissemination method shown in FIG. 10 can be used todisseminate CSI for MAT schemes using more than two rounds. For example,to enable a (r+1)-round transmission, eavesdropper CSI is required fromround-r transmissions. The CSI associated with the channel of theeavesdropper at the time a round-r message is transmitted issimultaneously needed by the r UEs that are the intended recipients ofthe round-r message.

FIG. 11 is a timing diagram illustrating an example with r=2, involving3 UEs, each having eavesdropped a round-2 message intended for the othertwo. FIG. 11 shows how each of three eavesdropper-CSI channels can bemade available to the pair of intended receivers by use of efficientscalar FB. Note that prior to the DL transmission, each UE has one ofthe coefficients and needs the other two. In the embodiment highlightedin FIG. 11, during time-frequency block (t^(DL), f^(DL)), the basestation transmits DL pilots from a single antenna element. These pilotsenable each of the 3 UEs to estimate their channels to thepilot-transmitting antenna. On a subset of resource elements duringtime-frequency block (t^(DL), f^(DL)), the base station also transmits(from the pilot-transmitting antenna element) two specific scalar linearcombinations of the three channel coefficients. The values of the twolinear combinations are depicted in the FIG. 11 as u₁, and u₂. Themultipliers of each channel coefficient transmitted in each of the twolinear combinations are made known a priori to each of the UEs. Each UEfirst recovers u₁, and u₂ and then removes from each value the componentthat is due to the channel coefficient it knows. Thus, the UE has twolinear equations of the two unknown channel coefficients. These twoequations enable the UE to estimate the two unknown channel coefficients(provided the coefficients in the two linear combinations of the threechannel coefficients are a priori properly chosen).

FIG. 12 is a timing diagram that illustrates a method for disseminatingfor round-1 eavesdropper CSIR in accordance with one embodiment.Referring to FIG. 12, the method exploits DL MU-MIMO transmission basedon outdated CSI in order to disseminate round-1 CSI needed for two-userMAT schemes between pairs of UEs. In this embodiment, the base stationneeds to have available at least 2 antenna elements to be able tosupport two-user MAT sessions. To disseminate round-1 CSIR, the methodrelies on mapping the CSIR dissemination problem into an intermediatestage of a “fictitious” 3-user MAT scheme, in which the eavesdropperCSIR that needs to be disseminated is “identified” as the eavesdroppermeasurements after round 1 of a 3-user MAT scheme.

In one embodiment, the protocol depicted in the timing diagram in FIG.12 uses two rounds of transmission. The first round includes threetransmission blocks, where the base station transmits “pair-wise” sumsof two 2-dimensional eavesdropper channels in each block. This operationrequires 2 antennas at the BS (although not shown in FIG. 12, DL pilotsare transmitted by the two antennas at the BS during each time-frequencyblock, which allow each UE to estimate the channel between the two BSantennas and the UE). The second round contains two scalar transmissions(and DL pilots from the single transmitting base station antenna).

Note that to enable the second round of the DL FB, eavesdropper CSIT isneeded from the first DL FB round. This can be provided with any of theearlier UL feedback methods in the invention. Given its own 5 scalarmeasurements, each UE can estimate the two 2-dimensional eavesdropperchannels it need. As a result, 5 channel uses can provide 12 scalarchannel coefficients in the downlink, for a net efficiency of 12/5 CSIRchannel coefficients delivered per channel use. In contrast the bestscalar DL FB technique depicted in FIG. 10 (which requires no CSITfeedback), this technique provides two channel coefficients per channeluse, while the reference scheme of FIG. 4 provides only one coefficientper channel use.

Scheduling Embodiments

Properly designed scheduling algorithms can provide significant benefitsin MU-MIMO schemes based on outdated CSI. The basic idea can beillustrated by use of considering enabling scheduling embodiments fortwo-user MU-MIMO schemes based on outdated CSI. In this case, for k=1,2, 3, . . . , R, the base station transmits to UE k a sequence oftwo-dimensional messages x_(k)(1), x_(k)(2), . . . ,x_(k)(n), . . .During the transmission of the n-th (round-1) message for user k,x_(k)(n), each user, j, with j=1, 2, 3, . . . , R, estimates itschannel, denoted by h_(j,k)(n), and saves the corresponding observation,that is, it saves a noisy version of h_(j,k)(n) x_(k)(n).

For comparison purposes with respect to the feedback methods set forthherein a baseline scheme is used in which the base station requests fromthe intended UE and from another L out (of the R−1) eavesdropping UEs tosend their CSI via UL feedback. Once the intended receiver channel forpacket x_(k)(n), i.e., h_(k,k)(n), and the L eavesdropper channels,i.e., {h_(j,k)(n)} for a subset (of size L) of j indices, becomeavailable at the base station, the base station can schedule aneavesdropper of the n-th (round-1) message for user k. This can beaccomplished by using techniques known in the art based on knowledge ofthe CSI of the intended user (i.e., user k) and the CSI of all the Leavesdroppers for which the base station has available eavesdropper CSI.

It is worth comparing the feedback requirements associated with thisbaseline scheme, which enables MU-MIMO transmission with schedulingbenefits, against those of a scheme that enables MU-MIMO transmissionwithout scheduling benefits. The CSIT cost of scheduling, i.e., the CSITfeedback overheads of the baseline design for scheduling call for L+1times as many coefficients to be fed back (CSIT) to the base station: Leavesdropper channels (as opposed to just one eavesdropper channel inthe scheme not relying on scheduling), as well as the channel of theintended receiver, k (which is not required at the base station whenscheduling is not employed). The CSIR costs associated with the baselinescheme for scheduling, however, are exactly the same as in thenon-scheduling case. This is because, only the channels of the scheduledeavesdroppers need to be made available to each intended receiver, i.e.,the same number of eavesdroppers as required by a CSIR disseminationscheme that enables MU-MIMO communication based on outdated CSI withoutscheduling benefits.

Embodiments of the invention enable scheduling benefits (in the sense ofimproved MU-MIMO performance by use of scheduling) with reduced CSITcosts (with respect to the reference CSI dissemination scheme enablingscheduling), at an additional cost in CSIR dissemination requirements.In one embodiment, this is accomplished by performing some of thescheduling decisions at each of the UEs. In one embodiment, first theintended-user channels, h_(k,k)(n), are made available to the basestation by use of UL feedback. This feedback phase can be enabled by thesame techniques used in non-scheduling scenarios, to feed backeavesdropper CSI to the base station (via UL transmissions).Subsequently, the base station employs DL transmissions to broadcastthese intended-user coefficients back to all UEs.

In one embodiment, for each j, UE j uses knowledge of the intended UE'schannel, h_(k,k)(n), and its own channel, h_(j,k)(n), to determinewhether or not UE j is a “good” eavesdropper for the n-th message of UEk, for any given n and for UE k, with k≠j. In one embodiment, given atUE j a possibly large set, N, of its own channels during (round-1)transmissions of messages intended for UE k with k≠j, that is, given{h_(j,k)(n)}_(1≦n≦N), UE j selects a fraction α≦1 of “sufficiently good”channels out of this set and schedules them for uplink transmissions inorder to provide feedback to the base station on the selected channels.

Methods for scheduling the “best” αN eavesdropped channels of user j onthe channels that packets were transmitted for user k can be readilyderived. In one embodiment, the eavesdropper channels{h_(j,k)(n)}_(1≦n≦N) are ordered via a quality metric which isindicative of incremental mutual information. For examples of qualitymetrics that can be used, see equations 29 and 30, and their supportingtext in Ansuman Adhikary, et al., “Multiuser MIMO with Outdated CSI:Training, Feedback and Scheduling,” in Proc. of the 49^(th) AllertonConference, September 2011. In one embodiment, the quality metricassociated with the n-th message for user k involves using the intendedreceiver channel, h_(k,k)(n), and the locally available eavesdropperchannel h_(j,k)(n), to estimate the mutual information increase thatwould be provided to the intended receiver, k, if the eavesdropperobservation of user j on the n-th message for user k were made availableto receiver k. In one embodiment, the eavesdropper scheduling techniquespresented in PCT Patent Application No. PCT/US2012/049612, titled “AMETHOD AND APPARATUS FOR SCHEDULING AND MU-MIMO TRANSMISSION BASED ONOUTDATED CHANNEL STATE INFORMATION AT THE TRANSMITTER”, filed Aug. 3,2012, for use at the base station to determine eavesdropper channelquality (and used for packet-centric scheduling), can be exploited atthe UE side, i.e., at UE j for choosing the αN channels on which UE j isto provide CSIT feedback.

The UE-based selective-eavesdropper CSI feedback embodiments describedherein provide reductions in the CSIT overheads with respect to thereference scheme, at the cost of additional CSIR feedback (theintended-user CSI needs to first be broadcasted by the base station toeach eavesdropper to enable user-terminal based eavesdropper channelselection for UL CSI feedback), potentially extra delays (e.g., forintended-user CSIT broadcasting, scheduling delays). Embodiments of theinvention described herein provide similar or even better performancethan the reference scheduling-scheme using L eavesdroppers per channel,and which have lower CSIT cost than the baseline eavesdropper CSIdissemination scheme that enables scheduling benefits.

In one embodiment, intended-receiver CSIT (available at the base stationvia UL feedback) is first made available to all users by broadcasting(in the downlink) the channel of each intended receiver, during thetransmission of a set of N messages for each user, for a set of R UEs(with R>L). In one embodiment, each UE, j, in the set of R UEs selects afraction α_(j,k)≦1 of sufficiently good eavesdropped channels ofmessages intended for user k, and schedules them for UL CSIT feedback,along with the corresponding round-1 message-identifying indices(signifying the user and packet index associated with the channelcoefficient fed back). In one embodiment, the α_(j,k)'s arepredetermined and fixed. In one embodiment, the set of {α_(j,k)}_(k≠j)are locally determined at each node j. In one embodiment, the userterminal selects the best α(R−1)N channels that it experienced over allthe messages for all the users over which the user is an eavesdropper.In that case, the α_(j,k) value equals one over N times the number ofthe α(R−1)N selected eavesdropper channels selected by user j, whichcorrespond to UE j channels during the transmission of messages by thebase station that were intended for user k. Examples of choosing valuesfor α are described in the context of specific embodiments that follow.In the embodiment described above, each user selects the intended-usermessages for which to send back eavesdropper CSIT independently of theeavesdropper-CSIT selection processes performed by other UEs. Ingeneral, the number of eavesdropper channels available to the basestation regarding the n-th packet of user k varies in such embodimentswith n and k, and it is equal to the number of eavesdropper UEs that(independently of one another) chose to and fed back CSI on the round-1transmission of the given message. Let ξ_(k)[n] denote the number ofeavesdropper channels available for the n-th channel of the k-th user.Note that

${\overset{\_}{\xi}}_{k} = {{\frac{1}{N}{\sum_{n = 1}^{N}\;{\xi_{k}\lbrack n\rbrack}}} = {\sum_{{1 \leq j \leq L};\;{j\bumpeq\not{}k}}\alpha_{j,k}}}$denotes the average number of eavesdroppers available to the basestation regarding each message for user k.

Clearly, ξ_(k)[n] can be as small as 0, corresponding to the case thatno eavesdropping UE fed back the channel it had experienced during thetransmission of the n-th message to user k, and as large as R−1,corresponding to the case that all eavesdroppers fed back their channelsfrom the transmission of the n-th message to user k. Having ξ_(k)[n]≧1allows the base station to schedule an eavesdropper for the n-th messageof user k. When ξ_(k)[n]>1, the scheduling schemes in PCT PatentApplication No. PCT/US2012/049612, titled “A METHOD AND APPARATUS FORSCHEDULING AND MU-MIMO TRANSMISSION BASED ON OUTDATED CHANNEL STATEINFORMATION AT THE TRANSMITTER”, filed Aug. 3, 2012, can be used toselect the “best” eavesdropper for packet x_(k)[n] out of the total ofξ_(k)[n] eavesdroppers.

When ξ_(k)[n]=1, in one embodiment, the base station schedules thesingle eavesdropper that fed back its channel. Note, however, thatscheduling (performance) benefits have already been harvested at the UEthat fed back their eavesdropper channel as this channel was ofsufficiently high quality to be fed back. For sufficiently large R, thescheduling performance gains provided by this selective feedback schemecan outperform those corresponding to a reference scheduling schemerelying on feedback by a fixed size set of L eavesdroppers forscheduling an eavesdropper.

When, however, ξ_(k)[n]=0, the base station has no eavesdropper channelsto consider for scheduling an eavesdropper for the n-th message of userk. Thus, additional feedback cycles are required in order to enableround-2 transmission (with or without scheduling benefits) involving then-th message of user k, for all values of n and k for which ξ_(k)[n]=0.

In one embodiment, the baseline scheme is applied (for enablingscheduling benefits) on all messages for which ξ_(k)[n]=0. In oneembodiment, the base station requests feedback from L eavesdroppers(e.g., at random) as for each message n of each user k, for whichξ_(k)[n]=0.

By properly choosing the fraction of the (eavesdropping) channels thateach UE feeds back to the base station, efficacy benefits can be enabledby this scheme without compromising its scheduling benefits. To see thisassume that each user has intended receiver CSI available for N messagesper user. Thus, user j has available eavesdropper CSI that it canpotentially send back on (R−1)N messages on which it is theeavesdropper. Assume that each user chooses a fraction α of thesechannels to send back. Let ζ_(j,k)[n]=1, if user j schedules and feedsback its channel h_(j,k)[n], and 0 otherwise. The probability thatζ_(j,k)[n]=1 is equal to α, as only a fraction cc of messages are fedback by user j. Note also that the number of eavesdropper channels ofthe n-th message of user k that are fed back via the UE-based selectiveCSI-feedback round satisfies

${\xi_{k}\lbrack n\rbrack} = {\sum\limits_{j\bumpeq\not{}k}\;{{\zeta_{j,k}\lbrack n\rbrack}.}}$The probability that ξ_(k)[n]=0 is thus (1−α)^(R-1). As a result, onaverage, for (1−α)^(R−1)NR out of the original NR user messages forwhich eavesdropper channels are needed at the base station, there are noavailable eavesdroppers at the base station after the completion of theUE-based selective CSI-feedback round. The second round of feedbackdissemination in this embodiment corresponds to using the referenceeavesdropper CSIT dissemination scheme. Thus, the average number ofeavesdropper channels feed back by this scheme over the two rounds(selective and reference rounds) is given by

$f_{{CSIT},{selective}} = {\frac{{R\;{\alpha\left( {R - 1} \right)}N} + {\left( {1 - a} \right)^{R - 1}{NRL}}}{NR} = {{\alpha\left( {R - 1} \right)} + {L\left( {1 - \alpha} \right)}^{R - 1}}}$

Given a value for R (number of users involved in this selectivedissemination scheme), and a value for the size of the eavesdropper setfor the baseline scheme, L, the optimal choice of α in terms ofminimizing f_(CSIT, selective), is given by

$\alpha^{*} = {1 - \left( \frac{1}{L} \right)^{\frac{1}{R - 2}}}$which, when substituted for a in the equation definingf_(CSIT, selective) above, yields the minimum value forf_(CSIT, selective) possible with this two-stage feedback disseminationscheme. It is worthwhile to compare the efficacy of this selective CSIfeedback scheme against the baseline scheme for a fixed L. The baselinescheme has a fixed cost of f_(CSIT, reference)=L eavesdropperchannels/message. For large R, it can be shown thatf_(CSIT, selective)(α*) approaches 1+log(L), thereby yielding benefitsof the order of L/log(L) in eavesdropper CSIT dissemination overheadswith respect to the baseline scheme. As a representative numericalexample, consider the case where it is required to design a system withscheduling performance equivalent (or better) to the performanceprovided by a baseline system that relies on scheduling based oneavesdropper feedback from L=10 users. The CSIT cost for the baselinesystem is f_(CSIT, reference)=L=10 eavesdropper channels/message. Takingas an example the case where R=20, the optimal value for the selectivedissemination scheme becomes α*≈0.0533, yielding an average CSITdissemination cost of value f_(CSIT, selective)≈3.16, i.e., a 3-foldimprovement over the reference method.

In one embodiment, a second round of selective feedback is employed toprovide additional CSIT feedback efficacy benefits. In one embodiment,each UE selectively feeds a α[1] fraction of its (eavesdropper)channels. Upon reception of selective CSIT eavesdropper feedback, thebase station signals back the indices of the user messages for whichξ_(k)[n]=0. Then, each user applies a second round of selective CSITfeedback and chooses a fraction α[2] of its (eavesdropper) channels onmessages for which ξ_(k)[n]=0 after round-one selective feedback. On theremaining messages for which the base station has no channels after thetwo rounds of selective signaling, in one embodiment, the base stationrequests and obtains feedback from L eavesdroppers per message, byrelying on the baseline scheme. It can be shown that, by optimizing α[1]and α[2] so as to reduce, and potentially minimize, the averageoverhead, this scheme has overhead that becomes 1+log(1+log(L)) forlarge R, yielding benefits of the order L/log(log(L)) over the baselineCSIT dissemination scheme, without compromising scheduling performance.

In one embodiment, a method for disseminating channel state information(CSI) coefficients from a first user terminal, where each CSIcoefficient corresponds to the CSI previously observed by the first userterminal during transmission resource blocks used by a base station tocommunicate information to a second set of one or more user terminals,comprises broadcasting by the base station the channel state information(CSI) of each user terminal in the second set of user terminals, on thesubset of transmission resources used by the base station to communicateinformation to each user terminal in the second set; receivingobservations of these transmissions by the first user terminal;performing user terminal based eavesdropper channel selection for uplinkCSI feedback to select a subset of the CSI coefficients for uplinktransmission; scheduling the subset of the CSI coefficients for uplinktransmission; and transmitting the subset of the CSI coefficients viathe uplink transmission.

FIG. 13 is a timing diagram associated with some embodiments of aUE-based selective eavesdropper CSIT feedback method. Referring to FIG.13, first, round-1 messages are transmitted by the base station for eachuser k. In particular, during time-frequency block (t_(k)(n), f_(k)(n)),the base station transmits a two-dimensional message x_(k)(n) and(orthogonal) DL pilots from two of its antennas. Then UE j estimates itschannel during the transmission of such a message based on the DLpilots, whether or not the message is intended for UE j (j=k) or not(k≠j). Subsequently, during time-frequency block (t^(UL-I), f^(UL-I))terminal j disseminates (by UL transmissions) its DL channels on DLtransmissions of messages intended for user j. This can be doneindependently via uplink transmissions dedicated to terminal j on inconjunction with uplink transmissions from other terminals. The basestation obtains observations of these transmissions and uses them toestimate the intended-user channels in round-1 message transmissions.Then the base station broadcasts these channels back to them, therebymaking them available to all UEs. In one embodiment, duringtime-frequency block (t^(DL-B), f^(DL-B)), the base station transmits(DL pilots and) these coefficients one at a time from one of itsantennas. Subsequently, UE j recovers (estimates) these intended-userterminal channel coefficients. Then UE j applies its own algorithm toselect a subset of its own CSI coefficients (in round-1 transmissions UEj was an eavesdropper) for each intended UE. Finally, UE j feeds back tothe base station (independently, or simultaneously with other UEs) theselected channel coefficients by use of UL transmissions duringtime-frequency block (t^(UL-R), f^(UL-R)). Finally, the base stationdetermines the subset of the messages for each UE on which it has noavailable CSIT and requests a subset of UEs to provide eavesdropper CSITon each such message.

FIG. 14 is a flow diagram of an embodiment of a process for sendingUE-based selective eavesdropper CSIT feedback at UE j, where UE jselects the best (according to a utility metric) α(R−1)N channels out ofthe (R−1)N channels that the UE has experienced during the transmissionof (R−1)N messages intended for other users. The process is performed byprocessing logic that may comprise hardware (circuitry, dedicated logic,etc.), software (such as is run on a general purpose computer system ora dedicated machine), or a combination of both.

In one embodiment, UE j has available its own (eavesdropper) channels{{h_(j,k)[n]}_(1≦n≦N)}_(j≠k), obtained (and stored) during thetransmission of (R−1)N messages intended for a set of R−1 users{{x_(k)[n]}_(1≦n≦N)}_(j≠k). In addition, UE j also has available theassociated intended receiver channels {{h_(k,k)[n]}_(1≦n≦N)}_(j≠k).These channels were made available to UE j via a broadcasting phase fromthe base station during which the set of channels{{h_(k,k)[n]}_(1≦n≦N)}_(1≦k≦R) were broadcasted in the downlink. Asshown in FIG. 14, UE j cycles through each intended UE index k≠j, one ata time, and each packet for that user, one at a time, in order tocompute and store a utility metric for each user-message combination.

Referring to FIG. 14, the process begins by processing logic in the UE jinitializing its utility-metric list as the empty list (processing block310 j). Then processing logic in UE j sets the intended user index kequal to 1 (processing block 320 j) and checks whether k equals j(processing block 330 j). In the event that k equals j, processing logicin UE j increments the value of k by 1 (processing block 335 j) andreturns to perform processing block 330 j again. In the event that kdoes not equal j, processing logic in UE j checks whether or not themaximum value of k has been exceeded (processing block 340 j).

If the maximum value of k=R has not been exceeded, processing logic inUE j sets the user-packet index n equal to 1 (processing block 345 j).Then processing logic in UE j fetches the intended-UE channel,h_(k,k)[n], and its own (eavesdropper) channel, h_(j,k)[n], from memory(processing block 350 j), then uses those channels to compute theassociated utility metric (processing block 355 j), and then appends thelocal list of utility metrics (processing block 360 j).

After appending the local list of utility metrics, UE j checks to see ifthe maximum message index value, n=N, has been reached (processing block365 j). If not, the UE increments the value of n by 1 (processing block370 j) and then proceeds in sequence through processing blocks 350 j-365j. If instead, in processing block 365 j, the maximum message indexvalue, n=N, has been reached, then processing logic in the UE incrementsthe intended-user index value of k by 1 (processing block 375 j) andreturns to perform processing block 330 j again.

Once the UE has cycled through all (R−1)N message-user index pairs,i.e., once the maximum value of k=R has been exceeded (processing block340 j), processing logic in UE j sorts the messages based on theirutility metric (processing block 380 j), selects the top α(R−1)Nmessage-user index pairs (385 j), and terminates its selection process.Then UE j proceeds to schedule its channels during the transmission ofthe selected α(R−1)N message-user index pairs for CSI feedback viauplink transmission.

FIG. 15 is a flow diagram of one embodiment of a process for feedingback UE-based selective eavesdropper CSIT feedback at UE j, where thefraction of the channels selected for CSIT feedback depends on the indexof the intended UE (the UE for which a round-1 message was intendedfor). The process is performed by processing logic that may comprisehardware (circuitry, dedicated logic, etc.), software (such as is run ona general purpose computer system or a dedicated machine), or acombination of both.

In FIG. 15, UE j selects the best (according to a utility metric)α_(j,k)N channels out of the N channels that the UE has experiencedduring the transmission of N messages intended for user k, and for eachk≠j.

Referring to FIG. 15, the process begins by processing logic in UE jsetting the intended user index k equal to 1 (processing block 420 j).Then UE j checks whether k equals j (processing block 430 j). In theevent that k equals j, UE j increments the value of k by 1 (processingblock 435 j) and returns to perform processing block 430 j again. In theevent that k does not equal j, processing logic in UE j checks whetheror not the maximum value of k has been exceeded (processing block 440j).

If the maximum value of k=R has been exceeded, the process terminates.If the maximum value of k=R has not been exceeded, however, processinglogic in UE j initializes its utility-metric list as the empty list(processing block 410 j), sets the user-packet index n equal to 1(processing block 445 j), fetches the intended-UE channel, h_(k,k)[n],and its own (eavesdropper) channel, h_(j,k)[n], from memory (processingblock 450 j), uses those channels to compute the associated utilitymetric (processing block 455 j), and then appends the local list ofutility metrics (processing block 460 j).

Thereafter, processing logic in UE j checks to see if the maximummessage index value, n=N, has been reached (processing block 465 j). Ifnot, processing logic in the UE increments the value of n by 1(processing block 470 j), and then proceeds in sequence throughprocessing blocks 450 j-465 j. If instead, in processing block 465 j,the maximum message index value, n=N, has been reached, processing logicin UE j sorts the messages of user k based on their utility metric(processing block 480 j) and selects the top α_(j,k)N message indicesfor user k for CSIT feedback. Then processing logic in UE j incrementsthe intended-user index value of k by 1 (processing block 475 j) andreturns to perform processing block 430 j again. Once the maximum valueof k=R has been exceeded (processing block 440 j), processing logic inUE j terminates its selection process and UE j proceeds to schedule itschannels during the transmission of the selected α_(j,k)N messageindices for user k for each for CSI feedback via uplink transmission.

Such UE-specific embodiments allow more control inperformance/CSIT-overhead trade-offs that can be enabled by theinvention. Consider for instance two UEs, k and m, and assume theaverage number of eavesdroppers available (or eavesdroppers able toprovide CSIT feedback) during the transmission of messages intended forUEs, k and m may be user specific. That is, there is an R_(m) and anR_(k) value, and R_(m)≠R_(k). Without loss of generality, consider thecase R_(m)>R_(k). Then, optimal the value of α is user-terminalspecific, i.e., there is two vales α_(k) and α_(m), and we haveα_(k)>α_(m). In one embodiment, the base station signals to each UE jwhether or not it is an eavesdropper for UE k. In one embodiment, UE jsets α_(j,k)=α_(k), and α_(j,k)=0, if the UE j is not required to senteavesdropper CSIT feedback on messages intended for UE k. In oneembodiment α_(j,k)'s are computed at terminal j, based on informationbroadcasted by the base station (e.g., the desired α_(k)'s) and locallyavailable information on the UE j. Consider for instance the case, whereN is eavesdropper-UE/intended-UE specific, i.e. where N_(j,k) denotesthe number of messages for user k on which UE j has available its own(eavesdropper) CSI available for feedback, and N_(j,k) depends on j andk. This locally available information at terminal j can be used inconjunction with information broadcasted by the base station (e.g., thedesired α_(k)'s) to determine at UE j values for its set of{α_(j,k)}_(k≠j).

Embodiments of the invention have one or more of the followingadvantages:

-   -   1) It enables efficient CSIT dissemination for enabling MU-MIMO        based on outdated CSI. Many of the embodiments are not possible        in conventional MU-MIMO due to the stringent timing requirements        in delivering CSIT.    -   2) It enables efficient CSIR dissemination for enabling MU-MIMO        based on outdated CSI. It allows higher efficiency CSIR        dissemination than pure broadcasting.    -   3) For the case that UEs can also listen in the uplink during        CSIT feedback transmissions by other UEs, the invention also        allows efficient dissemination of CSIT while also providing        partial CSIR dissemination to uplink-listening UEs, to be used        in conjunction with further DL CSIR transmissions. These methods        allow lowering further the number of transmission DL dimensions        for CSIR dissemination.

Whereas many alterations and modifications of the present invention willno doubt become apparent to a person of ordinary skill in the art afterhaving read the foregoing description, it is to be understood that anyparticular embodiment shown and described by way of illustration is inno way intended to be considered limiting. Therefore, references todetails of various embodiments are not intended to limit the scope ofthe claims which in themselves recite only those features regarded asessential to the invention.

We claim:
 1. A method for transmitting information in a multi-user(MU)-MIMO wireless communication system having a multi-antennatransmitter and a plurality of user terminals, the method comprising:obtaining, by a first user terminal, a first channel estimate of adownlink channel between a base station and the first user terminal fromat least one downlink pilot transmitted during a first time-frequencyblock with data intended for a second user terminal, different from thefirst user terminal; obtaining, by a second user terminal, a secondchannel estimate of the downlink channel between the base station andthe second user terminal from at least one downlink pilot transmittedduring a second time-frequency block with data intended for the firstuser terminal; sending the first and second channel estimates to thebase station; transmitting, by the base station during the thirdtime-frequency block, a linear combination of the first and secondchannel estimates received from the first and second user terminals. 2.The method defined in claim 1 further comprising: transmitting, by thebase station during the third time-frequency block that occurs after thebase station receives the first and second channel estimates, a pilotfrom at least one of the base-station antenna; and estimating, by thefirst and second user terminals, channels between the pilot-transmittingbase station antennas and the first and second user terminals.
 3. Themethod defined in claim 1 wherein the linear combination is a sum of twoscalar channel coefficients.
 4. The method defined in claim 1 furthercomprising: observing, by the first user terminal, the linearcombination; and obtaining, by the first user terminal, the secondchannel estimate based on the linear combination, the first channelestimate, and a channel estimate obtained at the user terminal from thepilot transmission during the third time-frequency block.
 5. The methoddefined in claim 3 wherein the second channel estimate comprisesestimates of channel coefficients.
 6. The method defined in claim 1wherein the first channel estimate is an estimate of the downlinkchannel between the base station and the first user terminal from atleast one downlink pilot transmitted during the first time-frequencyblock with data intended for a second user terminal and at least oneother user terminal, the at least one other terminal not being the firstuser terminal.
 7. The method defined in claim 1 wherein the linearcombination is transmitted on an Alamouti code.
 8. A multi-user(MU)-MIMO wireless communication system comprising: a base station witha multi-antenna transmitter; and a plurality of user terminals,including a first user terminal and a second user terminal, the seconduser terminal being different than the first user terminal, wherein thefirst user terminal is operable to obtain a first channel estimate of adownlink channel between the base station and the first user terminalfrom at least one downlink pilot transmitted during a firsttime-frequency block with data intended for the second user terminal,and the second user terminal is operable to obtain a second channelestimate of the downlink channel between the base station and the seconduser terminal from at least one downlink pilot transmitted during asecond time-frequency block with data intended for the first userterminal, the first and second user terminals being operable to send thefirst and second channel estimates, respectively, to the base station;and further wherein the base station is operable to: transmit, during athird time-frequency block that occurs after the base station receivesthe first and second channel estimates, a linear combination of thefirst and second channel estimates received from the first and seconduser terminals.
 9. The system defined in claim 8 wherein the linearcombination is a sum of two scalar channel coefficients.
 10. The systemdefined in claim 8 wherein the first user terminal is operable to:observe the linear combination; and obtain the second channel estimatebased on the linear combination, the first channel estimate, and achannel estimate between the base-station and first user terminalobtained from a pilot transmitted by the base-station during the thirdtime-frequency block.
 11. The system defined in claim 10 wherein thesecond channel estimate comprises estimates of channel coefficients. 12.The system defined in claim 8 wherein the first channel estimate is anestimate of the downlink channel between the base station and the firstuser terminal from at least one downlink pilot transmitted during thefirst time-frequency block with data intended for a second user terminaland at least one other user terminal, the at least one other terminalnot being the first user terminal.
 13. The system defined in claim 8wherein the base station is operable to transmit the linear combinationon an Alamouti code.